[cxx-interop] Improve performance of importing C++ namespaces

When ClangImporter calls Clang to import a module, Clang calls `SwiftLookupTableWriter::writeExtensionContents` for each namespace redecl, which called `addEntryToLookupTable` (a few frames omitted) which iterated over all of the namespace redecls, doing duplicate work. This caused the complexity to degrade from linear to quadratic.

`SwiftDeclConverter::VisitNamespaceDecl` is still called once per namespace, not per redecl, so we need to iterate over all the redecls there.

This dramatically improves the build time for SwiftCompilerSources when importing Clang headers in addition to LLVM headers.

Author: egorzhdan

lib/ClangImporter/ImportDecl.cpp

@@ -1129,11 +1129,13 @@ namespace {
       for (auto redecl : decl->redecls())
         Impl.ImportedDecls[{redecl, getVersion()}] = enumDecl;
 
-      // Because a namespaces's decl context is the bridging header, make sure
-      // we add them to the bridging header lookup table.
-      addEntryToLookupTable(*Impl.BridgingHeaderLookupTable,
-                            const_cast<clang::NamespaceDecl *>(decl),
-                            Impl.getNameImporter());
+      for (auto redecl : decl->redecls()) {
+        // Because a namespaces's decl context is the bridging header, make sure
+        // we add them to the bridging header lookup table.
+        addEntryToLookupTable(*Impl.BridgingHeaderLookupTable,
+                              const_cast<clang::NamespaceDecl *>(redecl),
+                              Impl.getNameImporter());
+      }
 
       return enumDecl;
     }

lib/ClangImporter/SwiftLookupTable.cpp

@@ -1960,21 +1960,21 @@ void importer::addEntryToLookupTable(SwiftLookupTable &table,
     llvm::SmallPtrSet<clang::Decl *, 8> alreadyAdded;
     alreadyAdded.insert(named->getCanonicalDecl());
 
-    for (auto redecl : named->redecls()) {
-      auto dc = cast<clang::DeclContext>(redecl);
-      for (auto member : dc->decls()) {
-        auto canonicalMember = member->getCanonicalDecl();
-        if (!alreadyAdded.insert(canonicalMember).second)
-          continue;
-
-        if (auto namedMember = dyn_cast<clang::NamedDecl>(canonicalMember)) {
-          // Make sure we're looking at the definition, otherwise, there won't
-          // be any members to add.
-          if (auto recordDecl = dyn_cast<clang::RecordDecl>(namedMember))
-            if (auto def = recordDecl->getDefinition())
-              namedMember = def;
-          addEntryToLookupTable(table, namedMember, nameImporter);
-        }
+    auto dc = cast<clang::DeclContext>(named);
+    for (auto member : dc->decls()) {
+      auto canonicalMember = isa<clang::NamespaceDecl>(member)
+                                 ? member
+                                 : member->getCanonicalDecl();
+      if (!alreadyAdded.insert(canonicalMember).second)
+        continue;
+
+      if (auto namedMember = dyn_cast<clang::NamedDecl>(canonicalMember)) {
+        // Make sure we're looking at the definition, otherwise, there won't
+        // be any members to add.
+        if (auto recordDecl = dyn_cast<clang::RecordDecl>(namedMember))
+          if (auto def = recordDecl->getDefinition())
+            namedMember = def;
+        addEntryToLookupTable(table, namedMember, nameImporter);
       }
     }
   }